The commercial preparation of phthalic acid anhydride (PAN) was accomplished first by the oxidation of naphthalene or o-xylene with chemical oxidants, then about 70 years ago the vapor phase oxidation of naphthalene and about 30 years ago by the vapor phase oxidation of o-xylene low in m- and p-xylene isomers. Both vapor phase oxidations use rather large volumes of air and solid particulate catalyst containing vanadium pentoxide. The general route devised for the recovery of commercially acceptable PAN product from the impure PAN produced by such vapor phase oxidation of naphthalene included the step of heating such impure PAN as a liquid, e.g., at a temperature in the range of from 130.degree. C. up to 285.degree. C., for up to 24 to 48 hours prior to a two step distillation sequence comprising simple total distillation followed by fractional distillation. The heating step converted oxidation coproducts which were color bodies or color formers into condensation products boiling at a temperature well above the boiling temperature of PAN. Hence such heat treating step was devised mainly to solve a product color problem but also to overcome product discoloration upon aging. It was found that the duration of the heat treating step could be substantially reduced by the use of chemical agents which promoted the condensation reactions. According to U.S. Pat. No. 3,407,216, impure PAN from vapor phase oxidation of naphthalene does not contain phthalide as an impurity.
After the use of o-xylene feeds containing not more than 15 percent m- and p-isomers had been demonstrated as a feasible feed for the vapor phase air oxidation to produce impure PAN, its purification used the established combination of heat treating followed by simple distillation and fractional distillation.
The time shortening color removal heat treating for impure PAN frm naphthalene or o-xylene feeds was suggested as being accomplished through the use of lithium and/or sodium nitrates (U.S. Pat. No. 2,512,283); carbonates, bicarbonates, sulfates or borates (U.S. Pat. No. 2,671,054); or phthalates added per se or formed in situ from the alkali metal hydroxides (U.S. Pat. No. 2,670,325). U.S. Pat. No. 3,155,688 specifically claims the use of potassium hydroxide and its salts with acids weaker than phthalic acid, as additive for color removal during heat treatment of impure PAN obtained by vapor phase oxidation of naphthalene. U.S. Pat. No. 2,670,325 specifically discloses the use of 0.05 weight percent sodium hydroxide as additive for color removing heat treatment of the impure PAN products obtained by vapor phase oxidation of naphthalene and o-xylene.
With respect to the removal of phthalide from PAN, we have reviewed the techniques for such removal as were brought to our attention by search reports resulting from searching conducted in Volumes 1 to 85 (1907 through 1976) of Chemical Abstracts, Derwent's Patent and Literature Abstracts (1964-1976) as well as pertinent information retrieved by machine search of the patents abstracted and indexed by Information For Industry of United States Patents granted from 1950 through 1975. We noted that the use of alkali metal hydroxides according to the technique of U.S. Pat. No. 2,670,325 was not there nor in any other publication found recognized as an agent useful for removing phthalide from impure PAN. We also noted that since phthalide is an intermediate incomplete oxidation product of o-xylene in the route to phthalic acid and/or phthalic anhydride, it is not surprising that the techniques previously proposed for removing phthalide from PAN involve the oxidation of phthalide.
Air or oxygen gas has been used to complete the oxidation of phthalide, preferably in the presence of a catalyst. Vanadium pentoxide catalyst is used in the heat treatment step or in the subsequent fractionation step in either the liquid or vapor phase according to Published German Patent Application (OLS) No. 1,935,008. The phthalide oxidation technique of French Pat. No. 1,600.064 involves the use of chromium, cobalt or vanadium with the impure PAN in an autoclave pressurized with oxygen or air to decrease the phthalide concentration to less than 0.1 weight percent. Liquid phase air oxidation of phthalide in impure PAN in the presence of cobalt, manganese and bromine is the phthalide removal technique taught by U.S. Pat. No. 3,208,423.
However, chemical oxidative removal of phthalide during the step of heat treating impure and discolored PAN has been an equally proposed technique. The use of sulfuric acid as oxidant in a particular mixture of phthalide, phthalic acid and phthalic anhydride is disclosed by U.S. Pat. No. 3,407,216. Japanese Published Patent Publication No. 10333/70 teaches the use during the heat treatment of impure PAN of potassium salts of different sulfur-containing acids; e.g., KHSO.sub.3, K.sub.2 S.sub.2 O.sub.5, KHS.sub.2 O.sub.4, K.sub.2 S.sub.2 O.sub.4, and K.sub.2 SO.sub.3. German Published Patent Application (OLS) No. 2,417,145 suggests adding a mixture of sodium carbonate and sodium nitrate to the impure PAN charged to the heat treating step. U.S. Pat. No. 3,338,924 discloses using a peroxide in combination with an alkali metal hydroxide and/or alkali metal carbonate during the heating treating of impure PAN to oxidize phthalide.